1. Field of the Invention
The present invention is directed to capsules used in the interstitial implantation of therapeutic radiation sources.
2. Description of the Prior Art
Advantages of interstitial implantation of radiation-emitting material for localized tumor treatment have been recognized for some time now. Interstitially implanted materials concentrate the radiation at the place where this treatment is needed, i.e., within a tumor so as to directly affect surrounding tumor tissue, while at the same time exposing normal tissue to far less radiation than does radiation that is beamed into the body from an external source.
One early implantable radioactive material was gold wire fragments enriched in radiation-emitting gold isotopes, such as gold-198. An advantage of gold wire, for interstitial implantation is that gold is compatible with the body in that it does not degrade or dissolve within the body. Another commonly used implantable material is radon-222.
Materials, such as gold-198 and radon-222, have significant counter-indicating characteristics for interstitial tumor treatment in that they emit relatively penetrating radiation, such as X-rays or gamma radiation of higher energy than is preferred, beta particles or alpha particles. Such materials not only subject the patient's normal tissue to more destructive radiation than is desired but expose medical personnel and other persons coming into contact with the patient to significant doses of potentially harmful radiation.
Capsules for enclosing the radioisotope material so as to form a fully assembled seed offer a number of important advantages. For example, it is important for medical health reasons that the radioisotope materials be isolated so as not to undergo a physical or chemical interaction with body fluids or body tissue while the seed is implanted in a living body. Further, the construction of the capsule should preferably allow rapid and easy insertion in the organ or body part to be treated, with minimal trauma to outer layers of tissue surrounding the affected part. For example, one popular technique for implanting seeds of this type is to inject the seeds into the body using a syringe or similar device. Due to the small size of the capsules, which frequently have outer diameters of the order of 0.5 mm to 0.8 mm, and lengths of the order of 5 mm, this technique is generally preferred.
U.S. Pat. No. 3,351,049 describes seeds including an enclosed outer shell which encases an X-ray-emitting isotope having a selected radiation spectrum. Notably, the isotope material comprises iodine-125 having a radiation spectrum which is quite favorable for interstitial use compared to previously used materials. The encasing shell localizes the radioactive iodine to the tumor treatment site, preventing the migration of iodine to other parts of the body, notably the thyroid, which would occur if bare iodine were directly placed in the tumor site. The use of an encasing shell permits the use of other X-ray-emitting isotopes which would dissolve in the body or present a toxic hazard to the recipient. Capsules containing iodine-125 have been used in treating patients.
U.S. Pat. No. 4,323,055 also discloses a radioactive iodine seed in which the radioisotope material is isolated by a container having sealed ends. As in the above-mentioned '049 patent, details of construction of the encasing shell are not given.
Another capsule construction for encasing radioisotope material is given in U.S. Pat. No. 4,510,924. A tubular body is disclosed having multiple layers, with an intermediate layer comprising the radioisotope material. The ends of the capsule are enclosed by flat disk-like members of a similar multilayer construction, which are welded at their outer edges to interior walls of the multiple-layered tubular body. A hollow interior is defined by the construction, to accommodate gases which may be generated over the operating life of the implanted seed. The multilayer, laminated design is difficult and costly to construct, especially considering the small size of the capsule. Further, the placement of active materials between layered walls of the capsule is not compatible with radioisotope materials that cannot be readily formed into sheet-like configurations. Some materials are more economically produced and function more effectively when configured in a three-dimensional form such as that of a sphere or egg-shaped body. Also, it is frequently desirable to provide an X-ray marker associated with the capsule to aid in the precise citing of the capsule within the body of a recipient. In general, X-ray markers, to be clearly identified, must often be provided in larger quantities than the radioactive material. However, it is generally desirable to incorporate the materials in as compact a package as is possible, a goal difficult, if not impossible, to achieve in the capsule of the above patent.
In order to function effectively, the radiation emitted from the radioisotope material must not be blocked or otherwise unduly attenuated. As indicated above, the small size of therapeutic seeds allows them to be inserted within the organ or tissue to be treated, so as to be totally surrounded thereby. Preferably, it is desirable that the radiation emitted from the radioisotope material have an equal distribution in all directions of emanation, i.e., have an isotropic radial distribution. In particular, it is generally desirable to avoid capsules with end constructions having a greater concentrations of radiation-absorbing material which obstructs the therapeutic radiation required for the successful treatment of affected tissues and organs.
Further, it is generally desirable to provide an economical capsule construction, using a fabrication technique which provides a hermetic seal without adversely affecting the capsule contents by heat or pressure.